The present invention relates to a bone fixing device used as a plastic surgery or orthopedic instrument for fixing a broken portion of a bone and extending or contracting a bone.
In general, in case of a medical treatment, a bone fixing device is used as an orthopedic instrument for fixing the broken portion of the bone. The bone fixing device is used not only for simply fixing the bone but also controlling the extension/retraction of the bone. In addition, the bone fixing device is used for adjusting the extension/retraction of the bone.
The present inventors et al. of Japanese Patent application No. 63-139245 filed on Jun. 8, 1988 relating to a bone fixing device having a fixture function. The fixing device is composed of the following components: a body frame having a linear rigidity and extending in an X-direction along the longitudinal direction of the bone; two holding frames positionally adjustably attached to the body frame for holding insert pins extending in a Z-direction perpendicular to the X-direction to be inserted into the bone; swing plates extending in a Y-direction perpendicular to the X-and Y-directions and swingable about the axis of Z-direction; pin heads mounted on the swing plates and swingable about the axis of Y-direction for detachably holding the insert pins; and a position adjusting mechanism for shifting at least one of the two holding frames on the body frame.
However, in the bone fixing device, in order to rotate the block-like swing plates about the Z-axis, it is necessary to juxtapose a plurality of rod-like members and to clamp the swing plates between the rod-like members. Also, in order to rotate the pin heads about the Y-axis, it is necessary to splice the pin heads with the tip ends of the swing plates by using shaft members.
Accordingly, the bone fixing device suffers from such problems that the physical size is increased and its weight is increased. As a result, a large amount of loads should be paid for treating the device.
On the other hand, there has been proposed a bone fixing device which is capable of extending/retracting the bone as shown in Japanese Patent application No. 63-333943 filed on Dec. 30, 1988 by the present inventors et al. This device is provided with two pairs of pin holding members for holding pins to be inserted into the broken bone. Also, this device is provided with a rod-like member composed, in combination, of a solid rod and a sleeve into which the solid rod is retractably or extendably inserted. Each of the pin holding members is fixed to the solid rod and the sleeve, respectively. Furthermore, a casing of an actuator is fixed to the sleeve, whereas a screw rod which is used as a reciprocation part of the actuator is connected to the solid rod.
With such an arrangement, when a motor of the actuator is operated, its rotational torque is transmitted in order through a reduction gear means, a bevel gear, a worm, and a worm wheel to a screw sleeve, and the screw rod is extended or retracted relative to the screw sleeve in a telescopic manner to thereby shift the pin holding member to adjust the extention/retraction of the fixed bone.
Such adjustment of extension/retraction of the bone is carried out by detecting a strain caused by the screw rod end abutting against a load cell provided at one end of the screw sleeve.
However, if the force applied to the bone would be detected by the relative movement between the screw rod and the screw sleeve as described above, there would be a fear that the value detected by the load cell would be not correct due to the factors such as a generation of a strain of the screw rod.
Also, since the rod member is of the inner and outer double type, there would be a fear that there is a positional offset between the pin holding members when the rod member is telescopically moved.